1. Field of the Invention
The present invention generally relates to the fabrication of masks used in the manufacture of electronic devices and, more particularly, to a method and system for making metal foil masks used to screen wiring patterns onto the substrate material of multilayer ceramic chip carriers.
2. Description of Related Art
Multi-layer ceramic substrates (MLC) are employed for making semiconductor devices. The fabrication of multi-layer ceramic chip carriers requires the ability to accurately transfer conductive pastes onto a substrate material. The most common method for the transfer of paste is a screening process, and a mask is used to precisely screen wiring patterns onto the substrate material. The openings of a high aspect ratio mask are fabricated with exacting dimensions to control the volume of material that is transferred to the substrate. A preferred mask construction is generated from thin metal films, such as Cu, Ni or Mo-based foils.
Ceramic wiring density has continually increased over time and design grids reduced to meet the ever-evolving technology of the silicon chip. The increasing amount of wiring on substrate layer places demands on the mask manufacturer. In order to achieve proper control of paste flow and volume, the mask must be very planar to the substrate surface to successfully gasket the conductive material. Any mechanical flaw of the mask surface can result in some amount of unwanted paste deposition. The percent open area on many of today's designs result in a mask that is rather fragile and highly subject to mechanical damage.
The process of building electroplated, thick-film, high aspect ratio metal masks involves several wet process steps, and has traditionally been a manually intensive operation. Operators had to transfer thin planar mask units between various process transport fixtures, with each fixture being specialized for a particular process machine. The continual transfer resulted in significant yield losses due to mechanical damage. Also, the demanding resist feature sizes required to generate the mask are easily de-bonded by any excess flex of the metal material.
While vacuum-flattening copper foil masks before securing them with adhesive to rigid frames has been attempted, this has not solved the problem of achieving a high degree of flatness in the foil.